1. Field of the Invention
This disclosure relates to flow measurements and, more particularly, to sonic flow measurement devices for flexible vessels.
2. Description of the Related Art
All metallic pipes may be considered sonic waveguides. This means that there is a mode of transmission of sonic energy, which preserves the relationship between Group and Phase velocities as sonic energy is transmitted down a pipe wall.
A transducer may be attached to a pipe such that the Phase velocity of sonic energy produced by the transducer is matched to a shear mode propagation velocity of the pipe material, and the transducer is operated at a frequency dependent on the round trip echo time in the radial direction. This will result in radiation of a coherent sonic wave into a liquid contained within the pipe at a constant angle to the pipe axis as the energy travels down the pipe. The sine of this angle is dependent, by Snell""s law, on the sonic propagation velocity of the liquid and the propagation velocity of the sonic energy down the wall of the pipe, since that energy travels axially, rather than at an angle within the pipe wall.
Since most non-metallic materials, such as plastics, do not support or only marginally support shear waves, pipes and tubes made of these materials do not typically exhibit waveguide properties. Accordingly, if sonic measurements are to be taken, it is not necessary for a sonic transducer to exhibit any particular Phase velocity or frequency to optimize the coherency of the liquid sonic beam. Unfortunately, the sonic impedance of plastic and other non-metallic materials are typically close to the impedance of most liquids. This is unlike metallic pipes in which the impedance is much higher than liquids. In sonic flow measurement systems, this results in a receive transducer obtaining sonic energy reflected from both the inner and outer pipe wall when operated with reflect mode transducers, which are desirable for crossflow correction. This reflection distorts the sonic signal with consequent miscalibration effects.
An additional problem presents itself when operating on plastic tubes, rather than metallic tubes. When operating on metallic tubes, a waveguide matched transducer may be excited to a waveguide mode of the tube or pipe at a location of a transmit transducer. This results in the injection of a Wide Beam signal, (i.e., waveguide matched), even though a limited aperture of the transmit transducer actually produces energy which is not, at the edges of the injection footprint, at a different phase velocity.
When clamped to a plastic pipe, however, these edges are not discriminated against. This results in the receive signal obtaining sonic signals that travel at a variety of angles in the liquid. The resultant multipath signals, each arrive at the receive transducer at a slightly different time, and cause further distortion and calibration instability.
Therefore, a need exists for an apparatus for accurately measuring flow in flexible vessels, such as plastic or rubber tubes and pipes.
An apparatus for measuring flow in flexible tubes, in accordance with the present invention, includes a housing including a first portion configured and dimensioned for receiving a first transducer and a second transducer therein and a second portion adapted to attach to the first portion to encapsulate a flexible tube between the first and second portion without cutting off flow within the tube. A plate is disposed within the housing in contact with the tube. The plate is sonically matched to the transducers to permit sonic energy transmitted from the first transducer to travel along the plate to provide sonic radiation from the plate to be received by the second transducer to measure flow characteristics within the tube.
In other embodiments, the first and second transducers may be disposed on a same side of the tube to operate in a reflect mode. A second sonically matched plate may be disposed on the same side of the tube as the first and second transducers to carry Sonically transmitted signals through the second plate to provide a reference signal.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.